Embodiments disclosed herein relate to a crosshead for a piston rod, specifically in a reciprocating compressor.
In the technical field of reciprocating machines (i. e. machines having a piston moving inside a cylinder such as, for example, internal combustion engines) the piston is usually connected to a crankshaft directly via a connecting rod. Specifically, the connecting rod is linked to the crankshaft via a crankpin. In this arrangement, however, the sideways forces from the connecting rod are transmitted directly to the piston, as the crankpin (and thus the direction the force is applied) moves from side to side with the rotary motion of the crankshaft.
These transverse forces are tolerable in a smaller reciprocating machine. However, in a larger one, such as for example a large reciprocating compressor, this would result in much greater forces, consequently causing an intolerable degree of wear on the piston and cylinder, as well as increasing overall friction.
Therefore, it becomes necessary to decouple the transverse movement of the crankpin from the axial movement of the piston. To perform this function, a crosshead links a piston rod (which is part of the piston) with a connecting rod (which is linked to the crankshaft via the crankpin). The sideways forces transmitted by the connecting rod are absorbed by the crosshead itself, thus allowing the piston rod to move along its axis with negligible transverse load.
Such crosshead is known in the state of the art. The crosshead comprises a main body having a first end facing the crankshaft and a second end facing the piston. A connecting rod is hinged on the first end, thus connecting it to the crankshaft. A piston rod is attached to the second end. Specifically, the piston rod is engaged into a receptacle of a flange, and it is retained therein primarily by mechanical interference. The flange itself is bolted onto the second end of the main body. Additionally, crosshead is provided with sliding shoes attached to the main body and slidably engaged on an internal wall of the cylinder. The sliding shoes themselves absorb the above mentioned sideway forces, allowing the crosshead to maintain the alignment with respect to the longitudinal axis of the piston.
An aspect of the prior art crosshead is that it is particularly prone to failure on the flange, as critical stresses can build up in the flange, especially on the fillet area near the bolting holes.